A great number of compounds are known to be used as mold inhibitors and many of them are actually used in commercial applications. However, not all of the conventional mold inhibitors can be used in silver halide color photography. Only a limited number of the known mold inhibitors are capable of exhibiting the desired effect without adversely affecting the photographic properties of the material and the storage stability of the photographic image. As is generally known, image dyes used in color photographic materials are vulnerable to a hot and humid atmosphere because such atmosphere provides a favorable condition for mold growth which is harmful to the dye image. Furthermore, an excretion from the mold or fungi discolors the dye.
Photographic materials are usually stored as they are pasted onto a paper leaf in an album or onto a mount. Alternatively, they are held between slide frames or wrapped with Japanese paper which is then held between frames. In these methods, the glue or paper fibers provide a nutrient for the growth of fungi, and this problem is particularly easy to occur in a hot and humid atmosphere, causing the discoloration of an image dye, particularly a cyan dye.
Formalin, benzoic acid, citric acid and acetic acid have been used for many years as mold inhibitors in silver halide color photographic materials. However, these compounds do not always ensure satisfactory results in mold inhibition. On the contrary, some compounds provide a nutrient for mold growth, and the problem is particularly conspicuous when acetic acid or citric acid is used. If citric acid is used in a stabilizer, a photographic material processed with the stabilizer is highly sensitive to mold growth upon long-term standing.
Photographic Science & Engineering, Vol. 3, May-June 1959 shows on page 132 that while ten-odd mold inhibitors are available, only pentachlorophenol is effective in application to color photographic images. However, this compound is harmful to humans.
Other mold inhibitors known in the art include mucochromium compounds (see U.S. Pat. No. 2,226,183), hydantoin and its derivatives (see U.S. Pat. No. 2,762,708), carboxyalkylpentahalobenzenethiol (see U.S. Pat. No. 2,897,081) and cerium hydrochloride or nitrate (see U.S. Pat. No. 3,185,571). Other relevant prior art references are U.S. Pat. Nos. 2,663,639, 3,503,746, 3,542,810 and 3,778,276, British Patents Nos. 987,010 and 1,065,920, and Japanese Patent Public Disclosure No. 157244/1982. However, almost all of the compounds shown in these references are organic sterilizers which are either expensive or highly oxidizable or sublimable to lose their efficacy within a short period of storage. It is therefore necessary to develop a mold inhibitor that is non-toxic and permits a photographic material to be stored over a prolonged period while inhibiting the mold growth.
Common recommendations in color photography are that dye images should be used under acidic conditions. However, if known organic acids are used as mold inhibitors, they become a source of nutrients for mold growth and impair rather than improve the stabily of dye images. This is another reason for the strong need for the development of an image stabilizer that exhibits its desired function effectively without causing mold growth in a photographic material.
The term "stabilization" of a silver image is often used in the processing of color photographic materials. This is the technique of fixing a not-to-be-washed black-and-white image, particularly silver image, and is shown in T. H. James; "The Theory of the Photographic Process", 4th Ed. Macmillan Publishing Co., Inc., p. 444. Several patent applications have been filed since 1943 concerning improvements of this technique; see, for example, British Patent No. 589,560 and U.S. Pat. Nos. 2,453,346, 2,453,347 and 2,448,857. Around 1965, a method was proposed for using ammonium thiocyanate to stabilize a silver image formed on the black-and-white photographic material. However, the processing method of the present invention is to stabilize color images formed on the color photographic material, and, therefore, is entirely different in technical idea from the method mentioned above.